As the number of revolutions of the main spindle of machine tools is made to increase, it will become necessary to increase the grabbing force (holding force) of chucks relative to the work. That is, due to the centrifugal force of the jaw of the chuck, part of grabbing force will be deleted by centrifugal force and finally cutting becomes an impossibility.
Such a holding device of conventional chucks is as shown in FIG. 1 to FIG. 4. That is, FIG. 1 represents a sectional profile of the upper part of the chuck device, in which radial slot grooves are provided on the body 1, divided in three equal parts in the radial direction, with master jaw 2, as shown in FIG. 2 slidably fitted. And at the front of the master jaw 2, top jaw 3 is secured with a bolt, and at the same time, in the inner diameter of body 1, plunger 4 is fitted slidably in the axial direction. Also, at the outer periphery of plunger 4, inverted T-shaped wedge groove 5 as shown in FIG. 3 is provided, splitting the periphery into three equal part directions (only one direction is shown), engaging wedge part 2a provided in the master jaw 2, which arrangement is shown in detail in the sectional cross section on X--X' line in FIG. 1.
On wedge part 2a of jaw 2, the force of arrow head marked P is applied as jaw 4 slides, but, since movement is possible also in the direction orthogonal to such force P, that is, in the XY direction, if effects of the holding surface (irregularity, etc.) of work are given during holding, then movement will be made from time to time in the XY direction by deformation of the neck portion of the wedge part 2a and besides, on the jaw area 5b (explanation given only for one side; the opposite side will be similar and explanation will be omitted hereunder) of the inverted T-shaped wedge groove provided on plunger 4, bending moment M is applied in the direction marked by the arrow head with part Q where contact between the inner periphery of body 1 makes or breaks servicing as base point; moreover as such applied force P is increased, mainly on part H from the upper corner part within the groove to the plunger outer periphery stress concentration will occur resulting in zigzag shaped damages.
In order to prevent the above-mentioned damages, it will serve the purpose if especially the above-mentioned part, that is, width H part shown in the figure is designed to be comparatively larger. But, this makes the cubic capacity of plunger 4 increase which will be opposite to the principle of compactness and also it will be a problem by increasing costs.
The present applicant has previously proposed U.S. Pat. No. 4,410,192 in order to solve these problems. Special features of the proposition were that, in a chuck having a chuck body, a plunger to slide in the axial direction of the body, and a master jaw linked to such axial direction movement and sliding in its radial direction, the master jaw is freely slidable, guided by the guide groove provided on the body. At the same time, the wedge like T-shaped projection is provided on the master jaw, such T-shaped projection being made to fit in the T-shaped wedge groove provided on the plunger, and moreover, it is so designed that when the plunger is made to move in the axial direction, the OPEN/CLOSE of the master jaw will be actuated by wedge action; besides master jaw guide groove width E on the body is constructed such that it will have the relationship of E&lt;G relative to wedge width G of the T-shaped projection of the master jaw.